1. Field of the Invention
The present invention relates to devices and methods for anchoring objects in concrete or other massive, rigid structures which are accessible only from a single surface.
2. Description of the Prior Art
For many years, it has been desirable to anchor objects, such as mounting plates, heavy machinery and other articles firmly in position upon a rigid, massive surface, such as a concrete floor. While the same problems of attachment occur in brick, stone, and other massive rigid materials, concrete is most frequently used as a floor support because of its relative ease of installation and because it can be cast in large sections.
In the past, the most common type of concrete anchor has been a device which is secured to the concrete in an anchor shaft by means of frictional engagement against the walls of the shaft. While several versions of such attachment devices exist, the most commonly used concrete anchor is an article called a "redhead". A redhead anchor is constructed of two portions: an elongated interiorally threaded cylindrical annular core, and a conical pin directed into one end of the core. The redhead is installed by boring an anchor shaft with a masonry drill to a depth just slightly in excess of the overall length of the redhead core. The core is then driven into the shaft from one end with the conical pin positioned in the lower end of the core with the conical apex directed upward. As the core is driven into the anchor shaft, the conical pin touches bottom and can move no further. Further driving force on the core drives the core downward toward the larger portion of the conical pin, thus creating a substantial outward force on the walls of the core as the pin is wedged into the lower end of the core. A threaded fastener, such as a bolt, may then be threadably engaged with the core from the upper end thereof to entrap a mounting plate, or the base of a heavy article of machinery in position on the concrete surface.
Distinct disadvantages exist with conventional "redhead" and other types of concrete anchors heretofore available. In the first instance, it is necessary to bore the anchor shaft to a relatively precise depth. If the shaft is too shallow, the upper extremity of the core will protrude above the concrete surface. If the shaft is to deep, a substantial gap will exist below the concrete surface and above the upper extremity of the core when the core is driven into position. This reduces the area of threaded engagement of the fastener in the core, and in extreme cases will preclude the fastener from engaging the core at all.
A further problem with conventional redhead devices is that the length of the fastener shank must be cordinated with the length of the redhead core. If the shank of a bolt fastener is too short, insufficient engagement will exist between the bolt shank and the core. If the bolt is to long, it cannot be seated properly in the core, since the apex of the conical redhead will interfere with the extreme end of the threaded shank of the bolt.
A further problem exists with conventional redhead concrete anchors in that they are sometimes dislodged by upward force on the core. Such upward force is frequently developed by the vibration of machinery held down by a fastener engaged in a redhead core or by direct upward force which may develop for a variety of reasons. When upward or outward force acts upon the head of the fastener, it is transmitted along the shank of the fastner as a tensile force tending to pull the core away from engagement with the conical pin. Even the slightest disengagement will reduce the radially outwardly directed forces on the walls of the core, and thus facilitate separation of the core from the redhead pin. It is thus entirely possible for the core portion of a redhead concrete anchor to be pulled entirely free of the pin and out of the anchor shaft.
A further problem with conventional devices is the requirement for an anchor shaft of relatively precise depth. In addition to the problems of interference with the fastener bolt shank and insecure engagement of the bolt shank with the anchor core, conventional concrete anchors can not always be easily removed when desired. Although such anchors sometimes effectuate undesired release from the concrete, at other times they are locked immovably in place. This represents a considerable inconvenience when the core has been damaged and is unsuitable for further use, or when a machine is to be moved, and it is desirable to remove the anchor and refill the anchor shaft. In such cases, the anchors are sometimes completely immobile and can be removed or partially removed only by laborious drilling from above.
While other types of conventional concrete anchors exist, other than "redheads", all such devices have one or more of the attendant disadvantages described above.